1) There is a new surfaceparm called surfaceparm ladder that is not available in baseq3. You can add this to a custom shader just like any other surfaceparm. See the Shader Manual in your GTK download if you're lost. Please note that every brush you apply this shader to will be climbable.
2) You can create an invisible, climbable area by applying the texture reaction/ladderclip to a brush. This brush will appear translucent in the editor so that you can see it. This makes it easy to make vines, walls, or anything else climbable. The ladderclip is available in the reaction texture directory. Since it's a shader, it will only appear in the texture window if you have followed the directions for adding reaction shaders on the Organizing Your Files page. See the screenshots below.
Method one is "cleaner" and more desirable for general use. There is no question about getting the ladder brush and climbable surfaces to line up because they one and the same. However, method two is required when you make models climbable (like modeled trees) or when your climbable surface is a lot of little brushes. Method two allows for more freedom in deciding what is climbable. Feel free to use both methods.
Here is a screenshot taken in GTK Radiant before the ladderclip was placed:
Here is the ladderclip:
